August 31, 2012

August 31st Is A Blue Moon…Or Is It?

August 31, 2012 will see a second full moon for the calendar month, commonly referred to as a Blue Moon. The first full moon for the month was seen on the night of August 1.

There are three different circumstances in popular culture in which a moon can be called "blue:" when the moon actually appears blue, when there are three full moons in a season, or like tonight, when there are two full moons in a calendar month.

The question is, are these definitions right? Let's look at each one.

A full moon that actually appears blue is very rare. It happens because of ash or dust in the air, sometimes from volcanic eruptions or major forest fires, which act like a color filter for your eyes making the moon appear blue. Because these types of blue moons only happen after a major event, they aren't predictable. There are a few other reasons the moon could appear blue, according to Les Cowley, an atmospheric optics expert. "Our eyes have automatic 'white balances' just like digital cameras. Go outdoors from a cozy cabin lit by an oil lamp (yellow light) and the moon will appear blue until your eyes adjust."

In 1883, though, people saw blue moons, green moons, lavender suns and even noctilucent clouds that glowed at night. Sounds impossible, right? The reason was Krakatoa, an Indonesian volcano that spewed ash to the top of Earth's atmosphere. The explosion was equivalent to a 100 megaton nuclear bomb and the atmospheric affects lasted for years. The ash plumes were filled with particles 1 micron wide, about the same wavelength as red light. The particle scattered the red light, allowing the blue to pass through, thus acting like a blue filter.

This happened again at least three more times in recent history: in 1980 after the eruption of Mt. St. Helens, in 1983 after the eruption of the El Chichon volcano in Mexico, and in 1991 after Mount Pinatubo blew.

There are examples of major forest fires causing a blue colored moon as well. The giant muskeg fire of September 1953 in Alberta, Canada produced lavender suns and blue Moons from North America to England. There are plenty of wild fires burning in the U.S. this month, so if any of them produce smoke with an extra dose of micron-sized particles, the full Moon on August 31 just might turn blue.

This is definitely a blue moon, in both the literal sense and in the sense of "something that happens very rarely," the original meaning of "once in a blue moon." There is no way to predict when this type of blue moon might happen as it takes some very large and catastrophic event to cause it. Strangely, this is not where the saying came from, however. A real blue moon was thought to be impossible, centuries ago when this phrase came into popular use, and that is what they meant when they said it. Impossible. "Once in a blue moon," "the moon is made of green cheese," and "when pigs fly" all were synonyms for something so absurd it couldn't possibly be real.

As a trivia side note, you are much more likely to see a red moon than a blue one. When the moon is hanging low in the sky, it often appears red for the same reason that sunsets do. The atmosphere is full of aerosols much smaller than the ones injected by volcanoes, which act to scatter blue light instead of red. For this reason, red blue moons are much more common than blue moons.

The second type of blue moon happens when there are four full moons in a season. A season generally only has three full moons. The third moon out of four is the blue moon, because it comes at an unusual time and doesn't fit the pattern of naming moons according to where they fall in relation to the solstices and equinoxes. From 1932 to 1957, the Maine Farmer's Almanac listed the blue moons that this convoluted seasonal rule created.

Another bit of trivia for you, according to the August 1937 issue of the Farmer's Almanac, this blue moon, or inconstant moon, is the reason behind our superstition about the number 13 being unlucky.

"However, occasionally the moon comes full thirteen times in a year. This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar. It became necessary for them to make a calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number."

By this definition, though, the moon on August 31 is NOT a blue moon. It is merely a second full moon this month. The last blue moon of this type appeared in November 2010, and the next will be August 2013.

The last definition for a blue moon is the one that matters for us this month. It occurs when there is a second full moon in one calendar month. The time between one full moon and the next is nearly the length of a full calendar month, approximately 29.5 days. So, sometimes, if a full moon happens in the first few days of a month, then a second full moon can happen at the end of the month. This type of blue moon comes around every two to three years.

Can there be more than one of this type of blue moon in a year? Yes, there can. In 1999, there were TWO blue moons and it will happen again in 2018.

The problem here is that this definition, though accepted in popular culture and even listed in the American Heritage Dictionary, is wrong.

"In modern usage, the second full Moon in a month has come to be called a 'Blue Moon.' But it's not!" says Kelly Beatty, Senior Contributing Editor for Sky & Telescope magazine. "This colorful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope back in March 1946, and it spread to the world from there."

Sky & Telescope admitted its error in 1999, about 53 years too late. By the time Canadian folklorist Phillip Hiscock and Texas astronomer David W. Olsen helped the magazine's editors figure out how the mistake was made, it had already spread into "common knowledge" and there it stays.

In 1946, a Sky & Telescope writer named James Hugh Pruett made an incorrect assumption about how the term was used in that Maine Farmer's Almanac we talked about earlier. The erroneous new definition was published and made its way into blooper history.

By the last two definitions, blue moons happen about every 2.7 years on average. The last time we had two full moons in a single calendar month was on New Year's Eve 2009, and the next one is scheduled to appear in July 2015.

So technically, August 31 is NOT a blue moon, by the strictest definition. Nevertheless, according to popular culture and the changing nature of the English language, it is.

"That's how the English language shifts. You can't beat back the tide," quips Sky & Telescope Senior Editor Alan MacRobert. "Not when the Moon is pulling the tide."

And finally, in perhaps a galactic sized coincidence, this blue moon is taking place on the day Neil Armstrong is laid to rest in a private service. Armstrong, the first man to walk on the moon, died last Saturday at the age of 82. His family suggests paying tribute to the American hero by looking up at the moon and giving a wink, perhaps this is the Universe's way of winking back.